The present invention relates to a method of determining the density of substrata of a considerable thickness by means of a gamma-radiation source comprising a collimator and of a detector comprising a collimator, both located on the surface of the substratum. The direction of radiation may be changed in relation to the direction of detection, and the change of the detected signal may be measured by means of the inventive method.
FIG. 3 illustrates an arrangement performing measurements on the basis of scattered gamma-radiation. The gamma-radiation emitted from the source is collimated to a thin beam, L, of gamma-quanta, which is damped exponentially during the passage of the object under measurement, said damping being dependent on the density of the object under measurement. The damping is substantially due to Compton-scattering, and the gamma-quanta are scattered in a direction away from the beam L in all directions. Some of the scattered quanta will--optionally subsequent to additional scatterings--be directed towards the detector.